Golf balls generally include a spherical outer surface with a plurality of dimples formed thereon. The dimples on a golf ball improve the aerodynamic characteristics of a golf ball and, therefore, golf ball manufacturers have researched dimple patterns, shape, volume, and cross-section in order to improve the aerodynamic performance of a golf ball. Determining specific dimple arrangements and dimple shapes that result in an aerodynamic advantage requires an understanding of how a golf ball travels through air.
When a golf ball travels through the air, the air surrounding the ball has different velocities and, thus, different pressures. The air develops a thin boundary layer adjacent to the ball's outer surface. The air exerts maximum pressure at a stagnation point on the front of the ball. The air then flows over the sides of the ball and has increased velocity and reduced pressure. The air separates from the surface of the ball at a top and a bottom separation point, leaving a large turbulent flow area called the wake that has low pressure. The difference in the high pressure in front of the ball and the low pressure behind the ball slows the ball down. This is the primary source of drag, which is the air resistance that acts on the golf ball in the direction opposite the ball's flight direction.
The dimples on a golf ball cause the thin boundary layer to flow in a turbulent manner. Rather than flowing in smooth, continuous layers (i.e., a laminar boundary layer), this turbulent boundary layer has a microscopic pattern of fluctuations and randomized flow. It is the circumference of each dimple, where the dimple wall drops away from the outer surface of the ball, which actually creates the turbulence in the boundary layer. The turbulence energizes the boundary layer and helps move the separation points further backward, so that the layer stays attached further along the ball's outer surface. As a result, there is a reduction in the area of the wake, increasing the average pressure behind the ball, and a substantial reduction in drag.
The shape of each dimple is also important in optimizing lift, which is an upward force on the ball that is created by a difference in pressure between the top of the ball and the bottom of the ball. This difference in pressure is created by a warp in the air flow that results from the ball's backspin. Due to the backspin, the top of the ball moves in the direction of the airflow, which shifts the top separation point to a location further backward. Conversely, the bottom of the ball moves against the air flow, which moves the bottom separation point forward. This asymmetrical separation creates an arch in the flow pattern that requires the air that flows over the top of the ball to move faster than the air that flows along the bottom of the ball. As a result, the air above the ball is at a lower pressure than the air underneath the ball. This pressure difference results in the overall force, called lift, which is exerted upwardly on the ball.
By using dimples to decrease drag and increase lift, almost every golf ball manufacturer has increased their golf ball flight distances. However, a golf ball must meet certain standards in order to be included on the official Conforming Golf Balls List (the “List”) produced by the United States Golf Association and The Royal and Ancient Golf Club of St. Andrews, Scotland, the two ruling bodies for the game of golf. Inclusion on the List is important for the commercial success of a golf ball, because it is a requirement for use in competitive golf, and because, even for recreational golf, most serious players won't use a ball unless it appears on the List.
One of the standards, commonly referred to as the “Symmetry Rule,” specifies that a ball must fly essentially the same distance and for essentially the same amount of time regardless of how it is oriented when struck by the golf club. It is important for a ball to have this property not only for inclusion on the List, but also to ensure consistent performance in use. If a ball flies farther when oriented in a certain way, it would cause the golfer to hit the ball farther than intended if the ball happened to be oriented that way before being struck. Commercial golf balls may fly differently in particular orientations, mostly due to asymmetry in the dimple pattern resulting from the inclusion of a straight dimple-free path around the equator of the ball. This path, or “parting line” or “great circle” was necessary to provide a place for the two halves of the mold to separate during the molding process. The effect was worsened by abrasive buffing that was performed on the parting line to remove flash and other molding artifacts. It was discovered that the effect could be minimized or eliminated by altering a group of dimples centered at the pole of each hemisphere, usually by making them shallower.
Seamless balls have been developed which use a corrugated or staggered parting line that weaves around the dimples to disguise its presence and minimize the disruption to the dimple pattern. Although it was believed that this type of parting line would improve symmetry of flight, it was found that seamless balls do not always display satisfactory symmetrical flight performance.
Using modified dimples in polar regions of seamless golf balls has been proposed as a means of improving symmetry, as disclosed, for example, in U.S. Patent Application Publication No. 2010/0240473, the entire disclosure of which is hereby incorporated herein by reference.